How To Troubleshoot Detector Loops

Recommended Steps to Test the Loop

1. Verify Detector Settings and Reset:
   • Check Sensitivity SettingsIf the detector has adjustable sensitivity settings (typically via DIP switches or dials) to control the detection threshold. If the sensitivity is set too high, it may falsely detect due to environmental noise or adjacent loops. Verify the sensitivity setting (consult the LMA-1800 manual for the appropriate range, typically 0 to 7 or similar). Reduce sensitivity temporarily to see if the false detection clears.
   • Reset the Detector: Press the reset button on the detector (or power cycle it) to clear any latched faults or false detections. Observe if the "Detect" LED persists after reset.
   • Check Mode Settings: Ensure the detector is in the correct mode (e.g., Presence or Pulse mode). Presence mode is typical for traffic signals, but a misconfigured mode could cause persistent detection.
2. Measure Loop Resistance:
   • Tools Needed: Digital multimeter (set to ohms).
   • Procedure: Disconnect the loop lead-in wires from the detector terminals in the cabinet. Measure the resistance between the two loop wires (loop pair).
     • Expected Value: A good loop typically has a resistance of 0.5 to 5 ohms, depending on loop size and lead-in length. Values significantly higher (e.g., >10 ohms) suggest a broken wire or poor splice; very low values (e.g., <0.5 ohms) confirm a short.
     • Action: If resistance is outside the expected range, inspect splices in pull boxes or the loop itself for damage (e.g., cuts, water ingress).
3. Measure Insulation Resistance (Loop to Ground):
   • Tools Needed: Megohmmeter (megger) set to 500V or 1000V.
   • Procedure: With the loop wires disconnected from the detector, measure the resistance between each loop wire and ground (e.g., cabinet ground or earth).
     • Expected Value: Insulation resistance should be very high, typically >100 megohms. Low values (e.g., <10 megohms) indicate insulation breakdown, possibly due to water ingress, damaged wire insulation, or a pinched cable.
     • Action: If insulation resistance is low, inspect the loop and lead-in for physical damage or poor sealing at splices. Replace or repair as needed.
4. Check Loop Inductance:
   • Tools Needed: Loop tester or LCR meter capable of measuring inductance.
   • Procedure: Disconnect the loop wires from the detector  and connect them to the LCR meter. Measure the inductance of the loop circuit (loop + lead-in).
     • Expected Value: Typical inductance for traffic loops is 20 to 300 microhenries (µH), depending on loop size and number of turns (e.g., a 6’x6’ loop with 3 turns is ~100–150 µH). The LMA-1800 operates within a broad inductance range, but extreme values may cause issues.
     • Action: If inductance is too high or low, check for incorrect loop turns, broken wires, or improper lead-in connections. Compare with design specifications for the loop.
5. Test for Crosstalk or Interference:
   • Issue: If multiple loops are nearby (common in flex lane setups), crosstalk between loops can cause false detections. The detector  has frequency settings to mitigate this.
   • Procedure: Check the frequency setting on the detector (via DIP switches or dials). Ensure adjacent loops are set to different frequencies.
     • Test: Temporarily disconnect adjacent loops and observe if the "Detect" LED clears. Alternatively, adjust the detector  frequency to a different setting and retest.
     • Action: If crosstalk is confirmed, permanently adjust frequencies to avoid overlap or rewire loops to increase physical separation.
6. Inspect Physical Connections:
   • Check Terminals: Ensure the loop lead-in wires are securely connected to the detectors terminals with no loose or corroded connections.
   • Inspect Splices: Open any pull boxes between the loop and the cabinet. Check splices for corrosion, water ingress, or poor connections. Ensure splices are sealed with waterproof connectors (e.g., epoxy or butyl rubber kits).
   • Examine Lead-In Cable: Look for physical damage to the lead-in cable (e.g., cuts, abrasions) that could cause intermittent faults.
7. Test Loop Continuity with a Loop Tester:
   • Tools Needed: Dedicated loop tester (e.g., EDI’s Loop Fault Tester or similar).
   • Procedure: Use a loop tester to simulate vehicle presence and measure loop response. These devices can confirm loop functionality and detect issues like unstable inductance or weak signals.
     • Action: If the tester indicates a fault, trace the issue to the loop or lead-in and repair as needed.
8. Environmental and External Factors:
   • Check for Metal Interference: Large metal objects near the loop (e.g., manhole covers, rebar) can cause false detections. Verify loop placement matches design specifications.
   • Check for Electrical Noise: Nearby power lines, transformers, or other electrical equipment can induce noise. Use a loop tester or oscilloscope to check for noise on the loop circuit.
     • Action: Relocate the loop if metal interference is an issue, or shield the lead-in cable (e.g., use twisted-pair lead-in) to reduce noise.
9. Verify Detector Operation:
   • Swap Detectors: If available, swap the detector with a known working unit and reconnect the loop. If the "Detect" LED no longer activates, the original detector may be faulty.
   • Test with a Known Good Loop: If possible, connect the detector to a different, verified loop circuit. If the issue persists, the detector itself may need repair or replacement.